Network Working Group R. Braden
Request for Comments: 1337 ISI
May 1992
TIME-WAIT Assassination Hazards in TCP
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This note describes some theoretically-possible failure modes for TCP
connections and discusses possible remedies. In particular, one very
simple fix is identified.
1. INTRODUCTION
Experiments to validate the recently-proposed TCP extensions [RFC-
1323] have led to the discovery of a new class of TCP failures, which
have been dubbed the "TIME-WAIT Assassination hazards". This note
describes these hazards, gives examples, and discusses possible
prevention measures.
The failures in question all result from old duplicate segments. In
brief, the TCP mechanisms to protect against old duplicate segments
are [RFC-793]:
(1) The 3-way handshake rejects old duplicate initial <SYN>
segments, avoiding the hazard of replaying a connection.
(2) Sequence numbers are used to reject old duplicate data and ACK
segments from the current incarnation of a given connection
(defined by a particular host and port pair). Sequence numbers
are also used to reject old duplicate <SYN,ACK> segments.
For very high-speed connections, Jacobson's PAWS ("Protect
Against Wrapped Sequences") mechanism [RFC-1323] effectively
extends the sequence numbers so wrap-around will not introduce a
hazard within the same incarnation.
(3) There are two mechanisms to avoid hazards due to old duplicate
segments from an earlier instance of the same connection; see
the Appendix to [RFC-1185] for details.
Braden [Page 1]

RFC 1337 TCP TIME-WAIT Hazards May 1992
For "short and slow" connections [RFC-1185], the clock-driven
ISN (initial sequence number) selection prevents the overlap of
the sequence spaces of the old and new incarnations [RFC-793].
(The algorithm used by Berkeley BSD TCP for stepping ISN
complicates the analysis slightly but does not change the
conclusions.)
(4) TIME-WAIT state removes the hazard of old duplicates for "fast"
or "long" connections, in which clock-driven ISN selection is
unable to prevent overlap of the old and new sequence spaces.
The TIME-WAIT delay allows all old duplicate segments time
enough to die in the Internet before the connection is reopened.
(5) After a system crash, the Quiet Time at system startup allows
old duplicates to disappear before any connections are opened.
Our new observation is that (4) is unreliable: TIME-WAIT state can be
prematurely terminated ("assassinated") by an old duplicate data or
ACK segment from the current or an earlier incarnation of the same
connection. We refer to this as "TIME-WAIT Assassination" (TWA).
Figure 1 shows an example of TIME-WAIT assassination. Segments 1-5
are copied exactly from Figure 13 of RFC-793, showing a normal close
handshake. Packets 5.1, 5.2, and 5.3 are an extension to this
sequence, illustrating TWA. Here 5.1 is *any* old segment that is
unacceptable to TCP A. It might be unacceptable because of its
sequence number or because of an old PAWS timestamp. In either case,
TCP A sends an ACK segment 5.2 for its current SND.NXT and RCV.NXT.
Since it has no state for this connection, TCP B reflects this as RST
segment 5.3, which assassinates the TIME-WAIT state at A!
Braden [Page 2]

RFC 1337 TCP TIME-WAIT Hazards May 1992
TCP A TCP B
1. ESTABLISHED ESTABLISHED
(Close)
2. FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
(Close)
4. TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
5.1. TIME-WAIT <-- <SEQ=255><ACK=33> ... old duplicate
5.2 TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> ????
5.3 CLOSED <-- <SEQ=301><CTL=RST> <-- ????
(prematurely)
Figure 1. TWA Example
Note that TWA is not at all an unlikely event if there are any
duplicate segments that may be delayed in the network. Furthermore,
TWA cannot be prevented by PAWS timestamps; the event may happen
within the same tick of the timestamp clock. TWA is a consequence of
TCP's half-open connection discovery mechanism (see pp 33-34 of
[RFC-793]), which is designed to clean up after a system crash.
2. The TWA Hazards
2.1 Introduction
If the connection is immediately reopened after a TWA event, the
new incarnation will be exposed to old duplicate segments (except
for the initial <SYN> segment, which is handled by the 3-way
handshake). There are three possible hazards that result:
H1. Old duplicate data may be accepted erroneously.
H2. The new connection may be de-synchronized, with the two ends
in permanent disagreement on the state. Following the spec
of RFC-793, this desynchronization results in an infinite ACK
Braden [Page 3]

RFC 1337 TCP TIME-WAIT Hazards May 1992
loop. (It might be reasonable to change this aspect of RFC-793 and kill the connection instead.)
This hazard results from acknowledging something that was not
sent. This may result from an old duplicate ACK or as a
side-effect of hazard H1.
H3. The new connection may die.
A duplicate segment (data or ACK) arriving in SYN-SENT state
may kill the new connection after it has apparently opened
successfully.
Each of these hazards requires that the seqence space of the new
connection overlap to some extent with the sequence space of the
previous incarnation. As noted above, this is only possible for
"fast" or "long" connections. Since these hazards all require the
coincidence of an old duplicate falling into a particular range of
new sequence numbers, they are much less probable than TWA itself.
TWA and the three hazards H1, H2, and H3 have been demonstrated on
a stock Sun OS 4.1.1 TCP running in an simulated environment that
massively duplicates segments. This environment is far more
hazardous than most real TCP's must cope with, and the conditions
were carefully tuned to create the necessary conditions for the
failures. However, these demonstrations are in effect an
existence proof for the hazards.
We now present example scenarios for each of these hazards. Each
scenario is assumed to follow immediately after a TWA event
terminated the previous incarnation of the same connection.
2.2 HAZARD H1: Acceptance of erroneous old duplicate data.
Without the protection of the TIME-WAIT delay, it is possible for
erroneous old duplicate data from the earlier incarnation to be
accepted. Figure 2 shows precisely how this might happen.
Braden [Page 4]

RFC 1337 TCP TIME-WAIT Hazards May 1992
TCP A TCP B
1. ESTABL. --> <SEQ=400><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
2. ESTABL. <-- <SEQ=101><ACK=500><CTL=ACK> <-- ESTABL.
3. (old dupl)...<SEQ=560><ACK=101><DATA=80><CTL=ACK> --> ESTABL.
4. ESTABL. <-- <SEQ=101><ACK=500><CTL=ACK> <-- ESTABL.
5. ESTABL. --> <SEQ=500><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
6. ... <SEQ=101><ACK=640><CTL=ACK> <-- ESTABL.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
7a. ESTABL. --> <SEQ=600><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
8a. ESTABL. <-- <SEQ=101><ACK=640><CTL=ACK> ...
9a. ESTABL. --> <SEQ=700><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
Figure 2: Accepting Erroneous Data
The connection has already been successfully reopened after the
assumed TWA event. Segment 1 is a normal data segment and segment
2 is the corresponding ACK segment. Old duplicate data segment 3
from the earlier incarnation happens to fall within the current
receive window, resulting in a duplicate ACK segment #4. The
erroneous data is queued and "lurks" in the TCP reassembly queue
until data segment 5 overlaps it. At that point, either 80 or 40
bytes of erroneous data is delivered to the user B; the choice
depends upon the particulars of the reassembly algorithm, which
may accept the first or the last duplicate data.
As a result, B sends segment 6, an ACK for sequence = 640, which
is 40 beyond any data sent by A. Assume for the present that this
ACK arrives at A *after* A has sent segment 7a, the next full data
segment. In that case, the ACK segment 8a acknowledges data that
has been sent, and the error goes undetected. Another possible
continuation after segment 6 leads to hazard H3, shown below.
2.3 HAZARD H2: De-synchronized Connection
This hazard may result either as a side effect of H1 or directly
from an old duplicate ACK that happens to be acceptable but
acknowledges something that has not been sent.
Braden [Page 5]

RFC 1337 TCP TIME-WAIT Hazards May 1992
The key to the failure in Figure 4 is that the RST segment 5 is
acceptable to TCP B in SYN-RECEIVED state, because the sequence
space of the earlier connection that produced this old duplicate
overlaps the new connection space. Thus, <SEQ=123> in segment #5
falls within TCP B's receive window [101,900). In experiments,
this failure mode was very easy to demonstrate. (Kurt Matthys has
pointed out that this scenario is time-dependent: if TCP A should
timeout and retransmit the initial SYN after segment 5 arrives and
before segment 6, then the open will complete successfully.)
3. Fixes for TWA Hazards
We discuss three possible fixes to TCP to avoid these hazards.
(F1) Ignore RST segments in TIME-WAIT state.
If the 2 minute MSL is enforced, this fix avoids all three
hazards.
This is the simplest fix. One could also argue that it is
formally the correct thing to do; since allowing time for old
duplicate segments to die is one of TIME-WAIT state's functions,
the state should not be truncated by a RST segment.
(F2) Use PAWS to avoid the hazards.
Suppose that the TCP ignores RST segments in TIME-WAIT state,
but only long enough to guarantee that the timestamp clocks on
both ends have ticked. Then the PAWS mechanism [RFC-1323] will
prevent old duplicate data segments from interfering with the
new incarnation, eliminating hazard H1. For reasons explained
below, however, it may not eliminate all old duplicate ACK
segments, so hazards H2 and H3 will still exist.
In the language of the TCP Extensions RFC [RFC-1323]:
When processing a RST bit in TIME-WAIT state:
If (Snd.TS.OK is off) or (Time.in.TW.state() >= W)
then enter the CLOSED state, delete the TCB,
drop the RST segment, and return.
else simply drop the RST segment and return.
Here "Time.in.TW.state()" is a function returning the elapsed
time since TIME-WAIT state was entered, and W is a constant that
is at least twice the longest possible period for timestamp
clocks, i.e., W = 2 secs [RFC-1323].
Braden [Page 7]

RFC 1337 TCP TIME-WAIT Hazards May 1992
This assumes that the timestamp clock at each end continues to
advance at a constant rate whether or not there are any open
connections. We do not have to consider what happens across a
system crash (e.g., the timestamp clock may jump randomly),
because of the assumed Quiet Time at system startup.
Once this change is in place, the initial timestamps that occur
on the SYN and {SYN,ACK} segments reopening the connection will
be larger than any timestamp on a segment from earlier
incarnations. As a result, the PAWS mechanism operating in the
new connection incarnation will avoid the H1 hazard, ie.
acceptance of old duplicate data.
The effectiveness of fix (F2) in preventing acceptance of old
duplicate data segments, i.e., hazard H1, has been demonstrated
in the Sun OS TCP mentioned earlier. Unfortunately, these tests
revealed a somewhat surprising fact: old duplicate ACKs from
the earlier incarnation can still slip past PAWS, so that (F2)
will not prevent failures H2 or H3. What happens is that TIME-
WAIT state effectively regenerates the timestamp of an old
duplicate ACK. That is, when an old duplicate arrives in TIME-
WAIT state, an extended TCP will send out its own ACK with a
timestamp option containing its CURRENT timestamp clock value.
If this happens immediately before the TWA mechanism kills
TIME-WAIT state, the result will be a "new old duplicate"
segment with a current timestamp that may pass the PAWS test on
the reopened connection.
Whether H2 and H3 are critical depends upon how often they
happen and what assumptions the applications make about TCP
semantics. In the case of the H3 hazard, merely trying the open
again is likely to succeed. Furthermore, many production TCPs
have (despite the advice of the researchers who developed TCP)
incorporated a "keep-alive" mechanism, which may kill
connections unnecessarily. The frequency of occurrence of H2
and H3 may well be much lower than keep-alive failures or
transient internet routing failures.
(F3) Use 64-bit Sequence Numbers
O'Malley and Peterson [RFC-1264] have suggested expansion of the
TCP sequence space to 64 bits as an alternative to PAWS for
avoiding the hazard of wrapped sequence numbers within the same
incarnation. It is worthwhile to inquire whether 64-bit
sequence numbers could be used to avoid the TWA hazards as well.
Using 64 bit sequence numbers would not prevent TWA - the early
termination of TIME-WAIT state. However, it appears that a
Braden [Page 8]

RFC 1337 TCP TIME-WAIT Hazards May 1992
combination of 64-bit sequence numbers with an appropriate
modification of the TCP parameters could defeat all of the TWA
hazards H1, H2, and H3. The basis for this is explained in an
appendix to this memo. In summary, it could be arranged that
the same sequence space would be reused only after a very long
period of time, so every connection would be "slow" and "short".
4. Conclusions
Of the three fixes described in the previous section, fix (F1),
ignoring RST segments in TIME-WAIT state, seems like the best short-
term solution. It is certainly the simplest. It would be very
desirable to do an extended test of this change in a production
environment, to ensure there is no unexpected bad effect of ignoring
RSTs in TIME-WAIT state.
Fix (F2) is more complex and is at best a partial fix. (F3), using
64-bit sequence numbers, would be a significant change in the
protocol, and its implications need to be thoroughly understood.
(F3) may turn out to be a long-term fix for the hazards discussed in
this note.
APPENDIX: Using 64-bit Sequence Numbers
This appendix provides a justification of our statement that 64-bit
sequence numbers could prevent the TWA hazards.
The theoretical ISN calculation used by TCP is:
ISN = (R*T) mod 2**n.
where T is the real time in seconds (from an arbitrary origin, fixed
when the system is started), R is a constant, currently 250 KBps, and
n = 32 is the size of the sequence number field.
The limitations of current TCP are established by n, R, and the
maximum segment lifetime MSL = 4 minutes. The shortest time Twrap to
wrap the sequence space is:
Twrap = (2**n)/r
where r is the maximum transfer rate. To avoid old duplicate
segments in the same connection, we require that Twrap > MSL (in
practice, we need Twrap >> MSL).
Braden [Page 9]

RFC 1337 TCP TIME-WAIT Hazards May 1992
The clock-driven ISN numbers wrap in time TwrapISN:
TwrapISN = (2**n)/R
For current TCP, TwrapISN = 4.55 hours.
The cases for old duplicates from previous connections can be divided
into four regions along two dimensions:
* Slow vs. fast connections, corresponding to r < R or r >= R.
* Short vs. long connections, corresponding to duration E <
TwrapISN or E >= TwrapISN.
On short slow connections, the clock-driven ISN selection rejects old
duplicates. For all other cases, the TIME-WAIT delay of 2*MSL is
required so old duplicates can expire before they infect a new
incarnation. This is discussed in detail in the Appendix to [RFC-
1185].
With this background, we can consider the effect of increasing n to
64. We would like to increase both R and TwrapISN far enough that
all connections will be short and slow, i.e., so that the clock-
driven ISN selection will reject all old duplicates. Put another
way, we want to every connection to have a unique chunk of the
seqence space. For this purpose, we need R larger than the maximum
foreseeable rate r, and TwrapISN greater than the longest foreseeable
connection duration E.
In fact, this appears feasible with n = 64 bits. Suppose that we use
R = 2**33 Bps; this is approximately 8 gigabytes per second, a
reasonable upper limit on throughput of a single TCP connection.
Then TwrapISN = 68 years, a reasonable upper limit on TCP connection
duration. Note that this particular choice of R corresponds to
incrementing the ISN by 2**32 every 0.5 seconds, as would happen with
the Berkeley BSD implementation of TCP. Then the low-order 32 bits
of a 64-bit ISN would always be exactly zero.
REFERENCES
[RFC-793] Postel, J., "Transmission Control Protocol", RFC-793,
USC/Information Sciences Institute, September 1981.
[RFC-1185] Jacobson, V., Braden, R., and Zhang, L., "TCP
Extension for High-Speed Paths", RFC-1185, Lawrence Berkeley Labs,
USC/Information Sciences Institute, and Xerox Palo Alto Research
Center, October 1990.
Braden [Page 10]